Coherent light is a common communications medium for the high speed transmission of large amounts of information. An optical receiver in such a communication system must be able to separate at high frequency over a large dynamic range (i.e., have a wide bandwidth) with a great linearity (i.e., have a low distortion). Such a receiver consists of two main parts' a photodetector and an amplifier. The typical photodetector used in these applications is a diode which is sensitive to light at a particular wavelength. This photodiode can be accurately modeled as an ideal current source in shunt with a capacitor, the capacitance of which is characteristic of the diode.
There are two common ways to amplify the signal from the photodiode. In FIG. 1A, photodiode 2 is coupled in series with load resistor 4 which matches the resistance of conventional amplifiers. Load resistor 4 has value of about 75 ohms for video systems and 50 ohms for microwave systems. Photodiode 2 is reverse biased by voltage source 5 which causes photodiode 2 to produce a current I.sub.I/A corresponding to the incident light signal 1. The current I.sub.I/A passes through load resistor 4 and the voltage signal generated across load resistor 4 is amplified using a conventional amplifier 3. Unfortunately, with most photodiodes, because their capacitance is small, the combination of the photodiode 2 with the load resistor 4 is unable to provide the desired carrier signal-to-noise ratio (.sup..about. 50db in a 1 GHz altogether bandwidth).
In FIG. 1B, a second receiver type called a transimpedance or transresistance amplifier is shown. Photodiode 7 is coupled to an amplifier input which has a very low impedance, practically a virtual ground. Photodiode 7 is reverse biased by voltage source 10 to generate a current I.sub.I/B analogous to the information from light signal 6. The current is input to amplifier 8 and a feedback resistor 9 is used to maintain stability. This configuration minimizes the effects of the capacitance of the photodiode and provides an amplifier with a wide bandwidth and a high gain. However, harmonic distortion severely limits receiver performance for many dynamic range applications such as the fiber optic transmission of cable television signals. See J. Hullett et al. "A Feedback Receiver Amplifier For Optical Transmission Systems,"IEEE Transactions On Communications, Oct. 1976 at pp. 1180-1185 for more information on receiver models.